It is a problem in the field of computer systems to provide an inexpensive, high performance, high reliability, and high capacity disk storage device. Traditional high performance and high capacity disk devices have typically used single large expensive disks (SLED) having form factors in the range of 12 or 14 inches.
The rapid acceptance of personal computers has created a market for inexpensive small form factor drives, such as 51/4, 31/2 inch, or smaller. Consequently, a disk storage device comprising a redundant array of inexpensive disks (RAID) has become a viable alternative for storing large amounts of data. Raid products substitute many small disk drives for a few very large expensive drives to provide higher storage capacities and throughput.
Striping is one well known technique used with RAID to improve I/O throughput. Striping involves the concurrent transfer of data to an array of disk drives in "stripes." For example, if the RAID has five disk drives, a stripe would consist of five blocks of data, and one block is transferred from each of the disk drives. In a five disk RAID, a data can typically be processed in about 1/5 the amount of time by transferring one block of data to each of the disk drives concurrently.
The drawback to replacing a single large disk drive with several small disks is reliability, since there is a much higher probability that one of the disk drives in the array will fail making the array inoperable. However, by means of data redundancy techniques, the reliability of RAID products can be substantially improved. Raid products typically use parity encoding to survive and recover from disk drive failures. Different levels of RAID organizations using parity encoding are currently known, see "A case for redundant arrays of inexpensive disks" David A. Patterson et al., Report No. UCB/CSD 87/891, December 1987, Computer Science Division (EECS), Berkeley, Calif. 94720. In RAID levels 4 and 5, one block of a stripe is reserved for parity data. RAID level 4 stores all parity blocks on the same drive, RAID level 5 distributes the parity blocks over all of the drives in the array.
Parity data are generally generated by using an exclusive or (XOR) function. RAID parity protection suffers from inherent problem that the number of I/O requests that must be serviced to maintain the parity data are many more than would be the case with non-RAID disks not using parity protection. For example, to write a block of new data to disk, the following steps must be performed: a) read the block storing the old data from the disk; b) read the block storing the old parity data from the disk) generate new parity data from the old data, the old parity data, and the new data; d) write the block storing the new data; and e) write the block storing the new parity data. In other words, the writing of a block of data in traditional RAID products typically requires four times the number I/O requests than would be the case with non-RAID disks.
Therefore, it is desirable to provide a system which reduces the number of I/O requests required to maintain parity data for the stripes of a RAID product.